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510                               CHEMICAL ENGINEERING
required in the case of pure magnesite this operation would be impossible under commercial conditions were it not for the presence of impurities, especially iron oxide, which bring about a certain degree of softening or sintering. In the Austrian magnesite the iron occurs in the natural condition, a fact which has made these ores so desirable in the past. In using the purer minerals the iron oxide must be added and ground with the raw rock before calcination. This applies particularly to the American magnesites.
The amount of iron, in the form of hematite ore, employed for this purpose is sufficient to bring the composition to that of the Austrian rock. The calcination is conducted in rotary kilns. Magnesite bricks are usually made from dead-burnt magnesite, ground quite fine, and mixed with a little water in the wet pan. The molding is accomplished by means of a powerful press. The bricks are then dried and burnt in down-draft kilns, being boxed in with silica brick, to a temperature about cone 18. The shrinkage in the firing of the bricks is about 0.125 in. per foot.
For mortar or cement used in building up furnace bottoms the ground, calcined magnesite is used, dampened with a little water. In the calcination of the commercial magnesite the mineral periclase is produced, sp. gr. 3.674. The completeness of the calcination may be judged from determination of the density. In addition to the principal mineral constituent, periclase, there are formed also accessory minerals as magnesium ferrite, MgO.Fe2Os; forsterite, 2MgO.SiO2j olivine, 2(MgFe)O.SiO2 and monticellite, CaO.MgO.SiO2. Magnesite of the purer grades has also been sintered or fused in the electric furnace and made into crucibles, tubes, etc., for laboratory use and research purposes. Thus, Burgess forms crucibles from crushed, sintered or fused magnesite, by electric heating in graphite molds, and Yensen by pressing pulverized, fused MgO with a small amount of magnesium hydrate in steel molds and heating the articles to 1,800C. in an electric furnace.
Properties of Magnesite Refractories.The chemical and mineralogical composition of the more impure materials has already beein indicated. From the nature of these refractories it is apparent that they will take up moisture from the air so that, according to Howe and McDowell, 2.5 per cent by weight may be absorbed after moistening the material daily, after 30 days. This hydration becomes more and more prominent with increase in the lime content with the result that the material may crumble and disintegrate, if the CaO is present in larger amounts. Thus, burnt dolomite may absorb 25 or more per cent of E^O. Magnesite refractories therefore should be kept in dry storage rooms.
The fusion point of pure magnesia has been estimated to be 2,800C. but that of the impure magnesite of commerce is about 2,165C., according to Kanolt.1 Washburn computes the latent heat of fusion to be 700 cal. per gram.
Upon reheating to temperatures up to 1,500C., an appreciable change in volume of up to 3 per cent may take place with well-burnt magnesite refractories. No figures seem to be available concerning the porosity of this material though it must be fairly high and between 25 to 30 per cent. The density of dead-burnt magnesite is about 3.5, that of fused MgO, 3.58. The bulk specific gravity of bricks is 2.6 to 2.75.
Magnesite bricks, subjected to a pressure of 50 Ib. per square inch, seem to fail suddenly at from 1,450 to 1,550C. through shearing. It has been suggested that this type of failure is due to a molecular change. It certainly cannot be ascribed to softening of the mass. Magnesite, however, becomes distinctly plastic at temperatures much below the fusion point.
The mean specific heat of pure magnesia is 0.234 at 61C. and 0.2762 at 390, mean temperatures. For magnesite bricks Heyn gives the values 0.208 at 0C. and 0.291 at 1,300, and Steger 0.225 between 20 and 200C.
i Tech. Paper No. 10, Bureau of Standards.